Final answer:
The cleavage of propeptides occurs outside the cell after fibrous collagen has been secreted, and it is a crucial step in converting procollagen into mature collagen.
Step-by-step explanation:
The step in the post-translational processing of fibrous collagen that occurs outside the cell, following secretion, is A. Cleavage of propeptides. This step is essential for converting procollagen into functional, mature collagen. Procollagen peptidases cleave the propeptides from the procollagen molecule, which then allows the collagen molecules to assemble into fibrils, the strong fibers found in connective tissues. This cleavage of propeptides does not take place in the cell but rather extracellularly after the procollagen has been secreted.
Once the cleavage is complete, the collagen molecules spontaneously assemble into the characteristic triple helix structure and cross-link to increase the tensility and strength of connective tissues. This step is quite distinct from other post-translational modifications such as the hydroxylation of proline, formation of the helix, and addition of sugar groups, which occur intracellularly.